1. Field of the Invention
The present invention relates to a frequency calibration loop circuit applicable to a frequency synthesizer and, more particularly, to a frequency calibration loop circuit capable of rapidly shifting the oscillation frequency of an oscillator to a target frequency band desired by a user.
2. Description of the Related Art
A frequency synthesizer is commonly used to generate a stable frequency for transmission and reception in the mobile communications sector.
A frequency calibration loop circuit enables the oscillation frequency of an oscillator in a broadband frequency synthesizer to be rapidly shifted to a target frequency band, thus shortening the time taken for the lock loop of the frequency synthesizer to lock.
As the related art digital frequency calibration loop circuit for broadband tuning, an adaptive frequency calibration loop circuit has been used. The adaptive frequency calibration loop circuit may include an oscillator, a main distributor, a frequency detector, and a state machine. The output frequency of the oscillator is controlled by an input bit, and the output frequency increases linearly according to an increase in a digital control value. The main distributor divides a waveform of the oscillation frequency output from the oscillator to generate a divisional signal. The frequency detector is configured as a counter that calculates the difference between the number of clocks of a reference frequency and that of a division frequency during an n clock of the reference frequency. The state machine receives the difference value of the clock numbers of the frequency detector during the n clock of the frequency period to determine a frequency state between the reference frequency and the division frequency to readjust the output bit. By repeatedly performing this process, the output frequency of the oscillator is shifted to a frequency by the product of the distribution value of the main distributor and the reference frequency.
However, the frequency calibration loop circuit readjusts the input bits of the oscillator by simply detecting the state of the frequency difference by means of the state machine. Thus, if the input bits for the frequency calibration of the oscillator are large, it takes an excessive amount of time to shift to a target frequency band.